The present invention generally relates to recording sheet transport apparatuses, and more particularly to a recording sheet transport apparatus of a duplex image forming apparatus such as a duplex copying machine and a duplex printer.
As is well known, when making a copy of a document, a copied image corresponding to the document image is usually formed on one side of a single recording sheet. With the recent progress in information society, the quantity of documents, data, publications and the like is greatly increasing, but it is uneconomical to copy such documents on one side of the recording sheets. There is now an increasing demand to copy documents on both sides of the recording sheets so as to reduce the total quantity of the recording sheets.
As one method of forming images on both sides of the recording sheet, there is the following proposed duplex recording system. According to this proposed duplex recording system, a reversing path is provided within an image forming apparatus. This reversing path comprises three stages of rollers, and a gate mechanism which is provided between the rollers of the first and second stages for switching a transport path. The roller of the second stage recedes so that a recording sheet can be transported within the reversing path while reversing and transporting an immediately preceding recording sheet. Hence, it is possible to reverse the recording sheets which are successively transported.
The recording sheets which are reversed are once stacked on an intermediate tray. After the image formation with respect to one side of the recording sheets is completed, the recording sheets accommodated in the intermediate tray are successively transported so as to carry out the image formation with respect to the other side of the recording sheets. As a result, the so-called duplex recording which carries out the image formation with respect to both sides of the recording sheets is carried out at a high speed. An image forming apparatus employing such a duplex recording system for making two-sided copies is proposed for example in a Japanese Published patent application No. 61-11864.
According to the duplex recording system, the following operation is carried out by the copying machine when copying identical image information on a predetermined number of recording sheets. In this case, an image forming part successively carries out the image formation with respect to a first side of the predetermined number of recording sheets and these recording sheets are once accommodated in the intermediate tray. Then, the recording sheets having the image information recorded on the first side are supplied from the intermediate tray to the image forming part so as to successively carry out the image formation with respect to a second side of the predetermined number of recording sheets. By successively carrying out the image formation first with respect to the first side of the recording recording sheets and then with respect to the second side of the recording recording sheets, it is possible to minimize a speed reduction of the image forming part in making consecutive copies.
But on the other hand, the above described duplex recording system suffers the following problems. That is, when the duplex recording system is applied to a laser printer which is provided with an image sensor capable of storing image information and both sides of a document are to be copied, for example, it is necessary to first obtain the image information of the last page in order to copy each page of the document in a correct sequence.
For example, when copying ten pages of image information on both sides of five recording sheets #1 through #5, it is necessary to obtain the image information of pages 10, 8, 6, 4, 2, 1, 3, 5, 7 and 9 in this sequence. In this case, the pages 10, 8, 6, 4 and 2 are recorded on the first side of the recording sheets #1 through #5 and the pages 1, 3, 5, 7 and 9 are recorded on the second side of the recording sheets #5 through #1. As a result, the recording sheet #5 is recorded with the pages 1 and 2, the recording sheet #4 is recorded with the pages 3 and 4, the recording sheet #3 is recorded with the pages 5 and 6, the recording sheet #2 is recorded with the pages 7 and 8, and the recording sheet #1 is recorded with the pages 9 and 10.
The need to obtain the image information of the last page first is especially a problem when an image memory is used in an exposure scanning system of the image forming apparatus. In particular, when the quantity of the image information becomes large, the image memory must store image information amounting to a large number of pages and it is impractical from the point of view of the high cost of the image memory which can store such a large quantity of image information. Accordingly, it is possible to limit the number of recording sheets which may be accommodated in the intermediate tray for the purpose of reducing the required capacity and thus the cost of the image memory. But in this case, the speed with which consecutive copies are made becomes reduced and it contradicts the original intention to speed up the making of the consecutive copies.
On the other hand, in order to eliminate the above described problems, there is a proposed image forming apparatus which supplies the recording sheets which are accommodated in the intermediate tray to the image forming part from the bottom of the stack. When copying ten pages of image information on both sides of five recording sheets #1 through #5, it is necessary to obtain the image information of pages 2, 4, 1, 6, 3, 8, 5, 10, 7 and 9 in this sequence. In this case, the pages 2 and 4 are recorded on the first side of the recording sheets #1 and #2, the page 1 is recorded on the second side of the recording sheet #1, and the page 6 is recorded on the first side of the recording sheet #3. Then, the page 3 is recorded on the second side of the recording sheet #2, the page 8 is recorded on the first side of the recording sheet # 4, and the page 5 is recorded on the second side of the recording sheet #3. Next, the page 10 is recorded on the first side of the recording sheet #5, the page 7 is recorded on the second side of the recording sheet #4, and the page 9 is recorded on the second side of the recording sheet #5. As a result, the recording sheet #1 is recorded with the pages 1 and 2, the recording sheet #2 is recorded with the pages 3 and 4, the recording sheet #3 is recorded with the pages 5 and 6, the recording sheet #4 is recorded with the pages 7 and 8, and the recording sheet #5 is recorded with the pages 9 and 10.
Therefore, the image memory need only store the pages 1 through 4 by the time the image formation of the page 1 ends, the pages 3 through 6 by the time the image formation of the page 6 ends, the pages 5 through 8 by the time the image formation of the page 8 ends, and the pages 7 through 10 by the time the image formation of the page 10 ends. As a result, the image memory only requires a memory capacity amounting to four pages, for example.
However, the following problems occur when making the two-sided copies in this manner. When making the two-sided copy, two recording sheets are not simply stacked on the intermediate tray but an operation of supplying a bottom recording sheet of the stack to the image forming part and an operation of stacking a recording sheet on the top of the stack are carried out simultaneously. For this reason, paper jams easily occur. The paper jams are caused when the recording sheet is supplied obliquely to an intended direction, two recording sheets are supplied at the same time and the like. When an attempt is made to eliminate the paper jams, the mechanism for supplying the recording sheet from the intermediate tray to the image forming part becomes complex and expensive. Furthermore, when the image forming apparatus uses recording sheets having different sizes, the intermediate tray must have a size in correspondence with a largest recording sheet which is used and the image forming apparatus itself becomes bulky.
In addition, in order to make the two-sided copy, the image forming apparatus must be provided with a driving device which drives the mechanism for reversing the recording sheet and a driving device which drives the mechanism for refeeding the recording sheet from the intermediate tray to the image forming part. As a result, the size of the image forming apparatus becomes large and the running cost thereof becomes high.
On the other hand, the recording sheet in a transport system which reverses the recording sheet and the recording sheet in the transport system which refeeds the recording sheet from the intermediate tray to the image forming part may be driven by a single driving source. In this case, the driving source selectively drives the recording sheet in one of the two transport systems, and a clutch mechanism or the like disengages the driving source from the other transport system. As a result, it is impossible to continuously transport the recording sheets in the two transport systems, and the cycle time required to make the two-sided copies may increase.